1. Field of the Invention
This invention relates to receiver protectors of the high power plasma/diode fast-acting limiter type.
2. Description of the Prior Art
Receiver protectors are used in the microwave duplexing art for permitting the use of a single antenna for transmission of high power output pulses and reception of low power received signals via a common microwave waveguide. Plasma within the receiver protector automatically inhibits the transmission of high-power wide-band-width electromagnetic wave energy in the direction from the antenna to the receiver thus preventing the transmitted pulse power from reaching and damaging the receiver. The receiver protector also permits the low-power received signal to come from the antenna through the microwave waveguide to be routed to the receiver.
The receiver protector may also have non-linear elements which function to short circuit the receiver when a high power pulse from the transmitter is routed to and reflected from the antenna and thus prevents a high energy pulse from damaging the receiver as described in an article entitled "What's New With Receiver Protectors?" by Harry Goldie in Microwaves, Vol. 15, No. 1, pp. 1-5. January 1976. The bandwidth of the receiver protector should equal or exceed the bandwidth of the transmitter pulse, otherwise, the out-of-bandwidth portion of the pulse will enter and possibly damage the receiver. In one known conventional receiver protector design, a number of diode limiters are placed across the output end of the receiver protector to prevent leakage of the transmitter pulse into the receiver input, which may result in burn out of the receiver input amplifier components. With the advent of transmitting pulses of extremely fast rise times on the order to 5 to 10 nanoseconds, the pulse waveform at the RF output of the receiver protector such as described in U.S. Pat. No. 4,027,255 issued on May 31, 1977 to H. K. Blakeney, et al. and assigned to assignee herein, may contain a bandwidth of several GHz which exceeds the in-band isolation bandwidth of the diode limiter section which is normally on the order of only 1 GHz. Consequently, the diode limiters cannot attenuate the "out-of-band" energy of the transmitted pulse. The fast risetime of the pulses causes high gap voltages to be generated across the gas discharge cones of this type of receiver protector during electron avalanching which results in a strong wideband noise energy burst being generated; the in-band energy is properly limited by the diode limiters whereas the out-of-band energy passed to the receiver becomes prominent. The out-of-band energy can exceed the safe leakage power of a sensitive low noise FET amplifier.
Measurements in the laboratory of a conventional receiver protector such as described in U.S. Pat. No. 4,027,255 showed that the transmitted output spike waveform contained a much wider band of frequencies, over 4 GHz, than the input test source contained. Investigation showed that the receiver protector itself generated a full waveguide band of frequencies and that the power amplitude of these frequencies exceeded the power amplitude of the in-band power pulse at the output port. In addition, the bandwidth of the diode limiter stage was only 1 GHz wide and thus did not attenuate the "out-of-band" energy. Such a problem was not expected to exist and was unknown prior to this receiver protector design and represented a new effect. This phenonema is caused by a combination of several factors: (a) fast rise (8 to 10 nanoseconds) incident pulses which cause high gap voltages across the gas discharge cones during discharge avalanching, resulting in strong wideband noise energy being generated; (b) in-band energy is properly limited due to the large number of varactor limiters (i.e., diode spike clippers) so that out-of-band energy becomes prominent (as seen at the output port on a spectrum analyzer set for 4 GHz bandwith) relative to in-band energy and, finally; (c) the safe leakage power for sensitive low noise FET amplifiers (NEC 388 manufactured by Nippon Electric Corp. of Japan; 0.5 micron gate-to-source length) is 100 milliwatts for 3 nanosecond spikes and 50 nw peak flat leakage which makes the amplifier subject to burnout when the excessive out-of-band energy leaks into it.
It was the combination of (a), (b) and (c) above which showed that a new type of receiver protector is required for receivers which use state-of-the-art low noise amplifier field-effect transistor (FET) front ends.
This invention provides an improvement to the commonly available type of receiver protector wherein its above-threshold bandwidth is increased sufficiently to inhibit leakage of high power transmitter pulses into receivers when said pulses contain frequencies which have a bandwidth wider than that of a conventional receiver protector.